Method and system for correcting for temperature variations in ultrasonic testing systems

ABSTRACT

A method and system for correcting for temperature variation in ultrasonic testing systems is disclosed. In one embodiment, an offset is determined for correcting a time of flight measurement, wherein the offset is based on the pulse delays at first and second calibration temperatures. In another embodiment, a factor is determined for correcting a time of flight measurement, wherein the factor is based on clock signal errors at first and second calibration temperatures.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to ultrasonic testing systems.

Several industries (e.g., oil and gas, refinery, chemical, power generation) require the transport of fluid through pipes. Nondestructive testing systems are placed on the outer surface of these pipes to monitor corrosion (or erosion) of the pipes, including corrosion on the interior of pipe walls. In some nondestructive testing systems, the probe or other nondestructive testing device is permanently coupled to the outer surface of the pipe to continuously monitor corrosion at that location to determine pipe corrosion rates and to determine whether that pipe location is in need of preventative maintenance to prevent a pipe failure. In other nondestructive testing systems, the probe is portable and can be moved along the outer surface of the pipe.

One example of a nondestructive testing system used to monitor corrosion of a pipe is an ultrasonic testing system. When conducting ultrasonic testing of a pipe, an ultrasonic signal is emitted from a probe coupled to the outer surface of the pipe and passed through the pipe. As the ultrasonic signal passes into and through the pipe, various reflections called echoes are reflected back to the probe as the ultrasonic signal interacts with the outer surface of the pipe, internal structures, voids or occlusions within the pipe, and with the inner surface (or back wall) of the pipe. The echo signals can be displayed on a screen with echo amplitudes appearing as vertical traces and time of flight or distance as horizontal traces. By tracking the time difference between the transmission of the ultrasonic signal and the receipt of the echoes, various characteristics of the pipe can be determined, including pipe thickness. Knowing the time of flight of the ultrasonic signal from the outer surface of the pipe to the inner surface of the pipe and then back to the outer surface of the pipe, as well as the speed of sound in the material that the pipe is made from (e.g., 5,800 m/s for stainless steel 316L) enables determination of the thickness of the pipe. If the thickness of the pipe at the location of the ultrasonic testing system decreases over time (e.g., as would be shown be a reduction in the time of flight of the back wall echo), this can be an indication of corrosion.

In order to make highly accurate thickness measurements, the time of flight measurements must also be highly accurate. The accuracy of time of flight measurements can be negatively impacted by performance variation caused by temperature variation of components of the ultrasonic testing unit. For example, the time (propagation) delay between the time that the logic circuit outputs a trigger to the pulser and the time that the ultrasonic signal is actually fired (pulse delay) varies based upon the temperature of the logic circuit (e.g., at lower temperatures, the time delay is shorter than the time delay at higher temperatures). Similarly, the frequency of the clock signal that determines, e.g., the timing of the sampling of the received ultrasonic signal or the time of flight measurement also varies based upon the temperature of the clock oscillator (e.g., at lower temperatures, the frequency is lower (the period is longer) than the frequency at higher temperatures). These temperature variations can result in different time of flight measurements depending on the temperature of the logic circuit and the clock oscillator that can result in inaccurate thickness measurements. For example, if the time of flight measured when the logic circuit is at +85° C. is 30 ns longer than the time of flight measured when the logic circuit is at −40° C., the thickness measurement at the higher temperature would be approximately 0.087 mm larger than at the lower temperature, even though the actual thickness had not changed.

In order to account for the differences in time of flight (and thickness) measurements resulting from performance variation caused by temperature variation in the logic circuits or clock oscillators, the ultrasonic testing system must be calibrated often. In those permanent installations where frequent calibration is not practical, heating or cooling devices can be installed in the ultrasonic testing system to maintain a consistent temperature for the components of the ultrasonic testing system, increasing the cost and energy demands of the inspection system.

The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE INVENTION

A method and system for correcting for temperature variation in ultrasonic testing systems is disclosed. In one embodiment, an offset is determined for correcting a time of flight measurement, wherein the offset is based on the pulse delays at first and second calibration temperatures. In another embodiment, a factor is determined for correcting a time of flight measurement, wherein the factor is based on clock signal errors at first and second calibration temperatures. An advantage that may be realized by the practice of some of the disclosed embodiments is more accurate time of flight measurements that correct for temperature variation of the ultrasonic testing unit. This will provide more accurate thickness measurements to more accurately identify corrosion.

In one embodiment, a method for correcting a measured time of flight measured by an ultrasonic testing unit for varying pulse delays caused by varying temperatures of the ultrasonic testing unit is disclosed. The method comprises the steps of calibrating the ultrasonic testing unit at a normalized calibration temperature by determining a normalized time of flight for a known material and a known thickness, determining a first pulse delay at a first calibration temperature of the ultrasonic testing unit, determining a second pulse delay at a second calibration temperature of the ultrasonic testing unit, and determining a time of flight pulse delay offset based on the difference between the second pulse delay and the first pulse delay and the difference between the second calibration temperature and the first calibration temperature.

In another embodiment, an ultrasonic testing system for inspecting a test object is disclosed. The ultrasonic testing system comprises an ultrasonic probe and an ultrasonic testing unit connected to the probe. The ultrasonic testing unit comprises a logic circuit, a pulser, a clock oscillator, a temperature measurement device, and a microcontroller comprising executable instructions for calibrating the ultrasonic testing unit at a normalized calibration temperature by determining a normalized time of flight for a known material and a known thickness, determining a first pulse delay at a first calibration temperature of the ultrasonic testing unit, determining a second pulse delay at a second calibration temperature of the ultrasonic testing unit, and determining a time of flight pulse delay offset based on the difference between the second pulse delay and the first pulse delay and the difference between the second calibration temperature and the first calibration temperature.

In yet another embodiment, a method for correcting a measured time of flight measured by an ultrasonic testing unit for varying clock signal frequencies caused by varying temperatures of the ultrasonic testing unit is disclosed. The method comprises the steps of calibrating the ultrasonic testing unit at a normalized calibration temperature by determining a normalized time of flight for a known material and a known thickness, determining a first time of flight for a first test piece of a known material and a first known thickness at a first calibration temperature of the ultrasonic testing unit, determining a second time of flight for the first test piece at a second calibration temperature of the ultrasonic testing unit, determining a clock signal error based on the difference between the second time of flight for the first test piece and the first time of flight for the first test piece, and determining a clock signal error factor based on the clock signal error and the difference between the second calibration temperature and the first calibration temperature.

This brief description of the invention is intended only to provide a brief overview of subject matter disclosed herein according to one or more illustrative embodiments, and does not serve as a guide to interpreting the claims or to define or limit the scope of the invention, which is defined only by the appended claims. This brief description is provided to introduce an illustrative selection of concepts in a simplified form that are further described below in the detailed description. This brief description is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features of the invention can be understood, a detailed description of the invention may be had by reference to certain embodiments, some of which are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only certain embodiments of this invention and are therefore not to be considered limiting of its scope, for the scope of the invention encompasses other equally effective embodiments. The drawings are not necessarily to scale, emphasis generally being placed upon illustrating the features of certain embodiments of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views. Thus, for further understanding of the invention, reference can be made to the following detailed description, read in connection with the drawings in which:

FIG. 1 is a schematic diagram of an exemplary ultrasonic testing system for inspecting a test object;

FIG. 2 is a graph showing exemplary variation in the pulse delays caused by variation in the temperature of the ultrasonic testing unit;

FIG. 3 is a flow diagram of an exemplary method for correcting a time of flight measured by an ultrasonic testing unit for varying pulse delays caused by varying temperatures of the ultrasonic testing unit;

FIG. 4 is a graph showing an exemplary plot of the pulse delays of FIG. 2 versus temperature;

FIG. 5 is a graph showing exemplary variation in the frequency (or period) of a clock signal caused by the variation in the temperature of the clock oscillator; and

FIG. 6 is a flow diagram of an exemplary method for correcting a time of flight measured by an ultrasonic testing unit for varying clock signal frequencies caused by varying temperatures of the ultrasonic testing unit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of an exemplary ultrasonic testing system 100 for inspecting a test object 10. The system 100 comprises an ultrasonic testing unit 120 connected to an ultrasonic probe 110 by an ultrasonic probe cable 112. The ultrasonic testing unit 120 comprises a pulser 122 that, after receiving a trigger signal from a logic circuit 128 (e.g., a field programmable gate array (FPGA) or an application specific logic circuit (ASIC)), transmits an analog electrical excitation pulse to drive the piezoelectric elements of the ultrasonic probe 110, causing an ultrasonic signal to be transmitted into the test object 10. The exemplary ultrasonic testing unit 120 also comprises a receiver/amplifier 124 for receiving and amplifying the analog electrical echo signal received back from the ultrasonic probe 110. A digitizer 126 is also provided for sampling the received analog electrical echo signal at a sampling rate determined by the frequency of the clock oscillator 132 (e.g., 50 MHz) and converting it to a digital signal that is input into the logic circuit 128, which can buffer the received digital signal and output the signal to one or more microcontrollers 130 for processing. The ultrasonic testing unit 120 can contain one or more temperature measurement devices 140 (e.g., thermocouples, thermistors, etc.) for measuring the temperature of the ultrasonic testing unit 120.

FIG. 2 is a graph showing exemplary variation in the pulse delays (and time of flights) caused by variation in the temperature of the ultrasonic testing unit 120 of FIG. 1 (e.g., individual components including the logic circuit 128). In one example, the logic circuit 128 is instructed to generate a trigger signal at the first rising edge 241 of the clock signal 240, establishing time zero 202. In this example, for illustrative purposes, the clock signal 240 has a frequency of 50 MHz, corresponding to a clock signal period of 20 ns. As shown in FIG. 2, the pulse delays 214, 224, 234 between time zero 202 and the actual time that the excitation pulses 212, 222, 232 are generated is determined, in part, by the temperature of, e.g., the logic circuit 128. When the temperature of the logic circuit 128 is lower, the pulse delay is shorter than when the temperature of the logic circuit 128 is higher. The pulse delays 214, 224, 234 then impact the time of flights 219, 229, 239 determined for the ultrasonic signals received from the ultrasonic probe 110 (FIG. 1).

Turning to the normalized electrical signal 210, the normalized pulse delay 214 for the normalized excitation pulse 212 triggered when the logic circuit 128 is operating at +25° C. (normalized calibration temperature) is 30 ns. This normalized pulse delay 214, in turn, delays the normalized time of flight 219 determined based on the normalized echo zero crossing 217 of the normalized echo 216 of the normalized electrical signal 210 (corresponding to the normalized ultrasonic signal echo), which is 150 ns. If the microcontroller 130 of the ultrasonic testing unit 120 determines time measurements only on the rising edges of the clock signal 240, the time of flight for the normalized electrical signal 210 would be determined based on the time of the eighth rising edge 248 of the clock signal 240, which corresponds to a time of 160 ns.

Turning to the first electrical signal 220, the first pulse delay 224 for the first excitation pulse 222 triggered when the logic circuit 128 is operating at −40° C. (first calibration temperature) is 15 ns. This first pulse delay 224, in turn, delays the first time of flight 229 determined based on the first echo zero crossing 227 of the first echo 226 of the first electrical signal 220 (corresponding to the first ultrasonic signal echo), which is 135 ns (15 ns shorter than the normalized time of flight 219). If the microcontroller 130 of the ultrasonic testing unit 120 determines time measurements only on the rising edges of the clock signal 240, the time of flight for the first electrical signal 220 would be determined based on the time of the seventh rising edge 247 of the clock signal 240, which corresponds to a time of 140 ns (20 ns shorter than the time of flight for the normalized electrical signal 210).

Turning to the second electrical signal 230, the second pulse delay 234 for the second excitation pulse 232 triggered when the logic circuit 128 is operating at +85° C. (second calibration temperature) is 45 ns. This second pulse delay 234, in turn, delays the second time of flight 239 determined based on the second echo zero crossing 237 of the second echo 236 of the second electrical signal 230 (corresponding to the second ultrasonic signal echo), which is 165 ns (15 ns longer than the normalized time of flight 219). If the microcontroller 130 of the ultrasonic testing unit 120 determines time measurements only on the rising edges of the clock signal 240, the time of flight for the second electrical signal 230 would be determined based on the time of the ninth rising edge 249 of the clock signal 240, which corresponds to a time of 180 ns (20 ns longer than the time of flight for the normalized electrical signal 210).

Without any correction, the variation in the time of flights 219, 229, 239 of the electrical signals 210, 220, 230 taken at different temperatures of the ultrasonic testing unit 120 can produce different thickness measurements for the test object 10 even though there has been no change in thickness, potentially failing to identify or falsely identifying corrosion of the test object. For example, assuming that the speed of sound in the material of the test object 10 is 5,800 m/s (e.g., for stainless steel 316L), the first thickness of the test object 10 corresponding to the first time of flight 229 (135 ns) will be 0.0435 mm thinner than the normalized thickness of the test object 10 corresponding to the normalized time of flight 219 (150 ns). Similarly, the second thickness of the test object 10 corresponding to the second time of flight 239 (165 ns) is 0.0435 mm thicker than the normalized thickness corresponding to the normalized time of flight 219 (150 ns). These varying thicknesses are a result of the inconsistent pulse delays between time zero 202 and the time when the excitation pulse is actually triggered.

FIG. 3 is a flow diagram of an exemplary method 300 performed by the ultrasonic testing unit (e.g., via executable instructions of the microcontroller 130) for correcting a time of flight measured by an ultrasonic testing unit 120 for varying pulse delays caused by varying temperatures of the ultrasonic testing unit 120 (e.g., the varying temperatures of the logic circuit 128). It will be understood that in some embodiments, the steps may be performed in a different order, certain steps will not be performed, and additional steps may be included.

At step 310, the ultrasonic testing unit 120 is calibrated at a normalized calibration temperature (T_(NC)) (e.g., +25° C.) to determine the normalized pulse delay 214 (PD₁=30 ns) and the normalized time of flight 229 (150 ns) produced at the normalized calibration temperature. While the illustrative embodiment has a normalized calibration temperature (T_(NC)) of +25° C., as shown for the normalized electrical signal 210 in FIG. 2, it will be understood that other normalized calibration temperatures can be employed (e.g., +22° C.). At the normalized calibration temperature, the ultrasonic testing unit 120 can be calibrated by determining the normalized time of flight for a test piece of a known material and a known thickness (e.g., 25 mm glass block) to produce the known thickness measurement at the normalized calibration temperature.

At step 320, and as shown in FIG. 2, the first pulse delay 224 (PD₁=15 ns) is determined with the ultrasonic testing unit 120 at a first calibration temperature (T_(C1)=−40° C.). At step 330, and as shown in FIG. 2, the second pulse delay 234 (PD₂=45 ns) is determined with the ultrasonic testing unit 120 at a second calibration temperature (T_(C2)=+85° C.). In one embodiment, the first calibration temperature is lower than the normalized calibration temperature (T_(C1)=+25° C.) causing the first pulse delay 224 to be shorter than the normalized pulse delay 214, while the second calibration temperature is higher than the normalized calibration temperature causing the second pulse delay 234 to be longer than the normalized pulse delay 214. It will be understood that, while the first and second calibration temperatures are different than the normalized calibration temperature in the exemplary embodiment, and the first and second pulse delays 224, 234 are different than the normalized pulse delay 214 in the exemplary embodiment, one of the first or second calibration temperatures could be the same as the normalized calibration temperature, and one of the first or second pulse delays 224, 234 could be the same as the normalized pulse delay 214. The pulse delays 224, 234 can be determined using an oscilloscope or similar device to determine the time between time zero 202 and the actual time that the excitation pulses 222, 232 are generated.

At step 340, a time of flight pulse delay offset (TOF_(PDOFFSET)) is determined based on the first pulse delay 224 (PD₁=15 ns), the first calibration temperature (T_(C1)=−40° C.), the second pulse delay 234 (PD₂=45 ns), and the second calibration temperature (T_(C3)=+85° C.). FIG. 4 is a graph showing an exemplary plot of the pulses 214 (30 ns), 224 (15 ns), 234 (45 ns) of FIG. 2 versus the calibration temperatures (+25° C., −40° C., +85° C.) of the ultrasonic testing unit 120. Based on the discovery that that there is a substantially linear relationship between the calibration temperature of the ultrasonic testing unit 120 and the resulting pulse delays 214, 224, 234, the time of flight pulse delay offset (TOF_(PDOFFSET)) can be determined as the slope of the time of flight pulse delay correction line 250 extending between, e.g., the first pulse delay 224 and the second pulse delay 234 on the graph of FIG. 4. Using the exemplary values presented above, the TOF_(PDOFFSET) is equal to +0.24 ns/° C. based on the following equation:

$\begin{matrix} {{T\; O\; F_{PDOFFSET}} = {\frac{\Delta\; P\; D}{\Delta\; T} = \frac{\left( {{P\; D_{2}} - {P\; D_{1}}} \right)}{\left( {T_{C\; 2} - T_{C\; 1}} \right)}}} & (1) \end{matrix}$

At step 350, a measured time of flight (TOF_(M)) is measured using the ultrasonic testing unit 120. At step 360, the temperature (e.g., T_(M)=+60° C.) of the ultrasonic testing unit 120 (e.g., the digitizer 126, the logic circuit 128, the clock oscillator 132, etc.) (FIG. 1) is measured by a temperature measurement device 140 of the ultrasonic testing unit 120. In one embodiment, the temperature measurement devices 140 can measure the temperature of one or more of the dies located on the printed circuit board of the ultrasonic testing unit 120 or the ground plane of the printed circuit board.

At step 370, the ultrasonic testing unit 120 can determine a pulser corrected time of flight (TOF_(PC)) based on the time of flight pulse delay offset (TOF_(PDOFFSET)) and the difference between the normalized calibration temperature (T_(NC)) and the measured temperature (T_(M)) in the following equation: TOF_(PC)=TOF_(M)+(T _(NC) −T _(M))*TOF_(PDOFFSET)  (2) Where TOF_(PC)=pulser corrected time of flight, TOF_(M)=measured time of flight, T_(NC)=normalized calibration temperature (e.g., +25° C.), T_(M)=measured temperature of the ultrasonic testing unit 120 (e.g., +60° C.), and TOF_(PDOFFSET)=time of flight pulse delay offset (e.g., +0.24 ns/° C.).

Using the exemplary values presented above, the pulser corrected time of flight (TOF_(PC)) for a measured temperature of +60° C. would be 8.4 ns shorter than the measured time of flight (TOF_(M)). As shown in FIG. 4, the point 254 on the time of flight pulse delay correction line 250 corresponding to a measured temperature (T_(M)) of +60° C. shows that, when the ultrasonic testing unit 120 is operating at a temperature of +60° C., the expected pulse delay would be about 8.4 ns longer than the normalized pulse delay 214 (30 ns) (FIG. 2) determined for the normalized calibration temperature, requiring that any measured time of flight taken at +60° C. be reduced by that offset amount of time 256. Similarly, referring to the point on the on the time of flight pulse delay correction line 250 corresponding to a measured temperature (T_(M)) of −15° C., when the ultrasonic testing unit 120 is operating at a measured temperature (T_(M)) of −15° C., the expected pulse delay 214 (21 ns) would be about 9.0 ns shorter than the normalized pulse delay (30 ns), requiring that any measured time of flight taken at −15° C. be increased by that offset amount of time 257.

In addition to causing variation in pulse delays, variations in the temperature of the ultrasonic testing unit 120, and in particular the clock oscillator 132 (FIG. 2), can cause variation in the frequency (or period) of the clock signal (e.g., at lower temperatures, the frequency is lower (the period is longer) than the frequency at higher temperatures). Since the time of flight is determined by counting the number of clock pulses between two points, variation in the frequency (or period) of the clock signal can produce different time of flight measurements at different temperatures. For example, the same time of flight duration may be counted as a greater number of clock pulses at a higher temperature than the number of clock pulses counted at a lower temperature.

FIG. 5 is a graph showing exemplary variation in the frequency (or period) of clock signals 410, 420, 430 caused by the variation in the temperature of the clock oscillator 132. In one example, the logic circuit 128 is instructed to generate an excitation pulse 442 at the first rising edges 411, 421, 431 of the clock signals 410, 420, 430, establishing time zero 402. In one embodiment, the time of flight 449 of the electrical signal 440 is determined by counting the number of clock pulses (e.g., by counting the clock signal rising edges) from time zero 402 to the echo zero crossing 447 of the echo 446. In this example, the ultrasonic testing unit 120 is configured for a clock signal having a frequency of 50 MHz, corresponding to a clock signal period of 20 ns. As shown in FIG. 5, because the clock signals 410, 420, 430 have different clock signal frequencies depending on the temperature of the ultrasonic testing unit 120, the time of flight 449 determined by counting the number of clock pulses will differ depending on temperature.

Turing to the normalized calibration clock signal 410 operating with the clock oscillator 132 of the ultrasonic testing unit 120 at a normalized calibration temperature (T_(NC)) of +25° C., the normalized calibration time of flight 449 determined by the ultrasonic testing unit 120 would be based on the twelfth rising edge 412 of the normalized calibration clock signal 410 (240 ns). Turning to the first clock signal 420 operating with the clock oscillator 132 of the ultrasonic testing unit 120 at a first clock temperature (T_(CL1)) of −40° C., the time of flight 449 determined by the ultrasonic testing unit 120 would be based on the ninth rising edge 422 of the first clock signal 420 (180 ns) (i.e., 60 ns shorter than the time of flight 449 determined based on the normalized calibration clock signal 410). Turning to the second clock signal 430 operating with the clock oscillator 132 of the ultrasonic testing unit 120 at a second clock temperature (T_(CL2)) of +85° C., the time of flight 449 determined by the ultrasonic testing unit 120 would be based on the eighteenth rising edge 432 of the second clock signal 430 (360 ns) (i.e., 120 ns longer than the time of flight 449 determined based on the normalized calibration clock signal 410).

Without any correction, the variation in the time of flight 449 determined based on the different frequencies of the clock signals 410, 420, 430 at different temperatures of the ultrasonic testing unit 120 can produce different thickness measurements for the test object 10 even though there has been no change in thickness, potentially failing to identify or falsely identifying corrosion (reduced thickness) of the test object. For example, assuming that the speed of sound in the material of the test object 10 is 5,800 m/s (e.g., for stainless steel 316L), the first thickness of the test object 10 corresponding to the time of flight 449 based on the first clock signal 420 (180 ns) will be 0.174 mm thinner than the normalized calibration thickness of the test object 10 corresponding to the time of flight 449 based on the normalized calibration clock signal (240 ns). Similarly, the second thickness of the test object 10 corresponding to the time of flight 449 based on the second clock signal 430 (360 ns) is 0.348 mm thicker than the normalized calibration thickness of the test object 10 corresponding to the normalized calibration time of flight 219 (240 ns). These varying thicknesses are a result of the inconsistent clock signal frequencies (i.e., the clock signal frequency of the normalized calibration clock signal 410 at +25° C. is greater than the clock signal frequency of the first clock signal 420 at −40° C., while the clock signal frequency of the normalized calibration clock signal 410 at +25° C. is less than the clock signal frequency of the second clock signal 420 at +85° C.).

FIG. 6 is a flow diagram of an exemplary method 600 performed by the ultrasonic testing unit (e.g., via executable instructions of the microcontroller 130) for correcting a time of flight measured by an ultrasonic testing unit 120 for varying clock signal frequencies caused by varying temperatures of the ultrasonic testing unit 120 (e.g., the varying temperatures of the clock oscillator 132). It will be understood that in some embodiments, the steps may be performed in a different order, certain steps will not be performed, and additional steps may be included. Since it has been determined that the variation in time of flight measurements caused by variation in the clock signal frequencies also depends upon the thickness of the test object 10, the clock signal frequency correction factor can be determined using two or more test pieces of known materials and known thicknesses (e.g., 25 mm, 50 mm, and 75 mm glass blocks) at two or more calibration temperatures (e.g., −40° C., +25° C., +85° C.) to determine the average percentage of error introduced in the time of flight measurements by the varying clock signal frequencies.

At step 610, the ultrasonic testing unit 120 is calibrated with the ultrasonic testing unit 120 at a normalized calibration temperature (T_(NC)) (e.g., +25° C.) to determine a normalized calibration time of flight (TOF_(NC)) (e.g., by counting the number of clock pulses) at the normalized calibration temperature. While the illustrative embodiment has a normalized calibration temperature (T_(NC)) of +25° C., it will be understood that other normalized calibration temperatures can be employed (e.g., +22° C.). This calibration step 610 can take place at the same time and at the same normalized calibration temperature as the calibration step 310 used for the pulse delay correction of FIG. 3. At this normalized calibration temperature, the ultrasonic testing unit 120 can be calibrated with a test piece of a known material (e.g., glass) and a known thickness to produce a first time of flight (i.e., known number of clock signal rising edges) at the normalized calibration temperature.

At step 620, a first time of flight (TOF₁) is determined for a first test piece of a known material and a first known thickness (e.g., first thickness (TH₁)=25 mm) with the ultrasonic testing unit 120 at a first calibration temperature (T_(C1)=−40° C.). At step 630, a second time of flight (TOF₂) is determined for the first test piece with the ultrasonic testing unit 120 at a second calibration temperature (T_(C2)=+85° C.). In one embodiment, the first calibration temperature is lower than the normalized calibration temperature (T_(NC)=+25° C.) causing the first time of flight to be shorter than the normalized calibration time of flight, while the second calibration temperature is higher than the normalized calibration temperature causing the second time of flight to be longer than the normalized calibration time of flight. It will be understood that, while the first and second calibration temperatures are different than the normalized calibration temperature in the exemplary embodiment, and the first and second time of flights are different than the normalized calibration time of flight in the exemplary embodiment, one of the first or second calibration temperatures could be the same as the normalized calibration temperature, and one of the first or second time of flights could be the same as the normalized calibration time of flight.

The first time of flight (TOF₁) can also be determined for a second test piece having a second thickness (e.g., TH₂=50 mm) and a third test piece having a third thickness (e.g., TH₃=75 mm) with the ultrasonic testing unit 120 at the first calibration temperature (T_(C1)=−40° C.). Similarly, the second time of flight (TOF₂) can also be determined for a second test piece having second thickness (e.g., TH₂=50 mm) and a third test piece having a third thickness (e.g., TH₃=75 mm) with the ultrasonic testing unit 120 at the second calibration temperature (T_(C2)=+85° C.). In one embodiment using three test pieces of different thicknesses, the following calibration data is determined, with the time of flights shown as the number of clock pulses:

TABLE 1 TOF₁ TOF_(NC) TOF₂ Test Piece (T_(C1) = −40° C.) (T_(NC) = +25° C.) (T_(C2) = +85° C.) 25 mm 6,000 6,005 6,010 (TH₁) 50 mm 12,000 12,020 12,040 (TH₂) 75 mm 18,000 18,045 18,090 (TH₃)

At step 640, for each test piece used with a different thickness, a clock signal error (CS_(ERR)) can be determined based on the difference between the second time of flight at the second calibration temperature and the first time of flight at the first calibration temperature, divided by the first or second time of flight. For each test piece used with a different thickness, the clock signal error (CS_(ERR)) can be determined by the following equation:

$\begin{matrix} {{C\; S_{ERR}} = {\frac{\Delta\; T\; O\; F}{T\; O\; F} = \frac{{T\; O\; F_{2}} - {T\; O\; F_{1}}}{T\; O\; F_{1/2}}}} & (3) \end{matrix}$

If multiple test pieces are used with different thicknesses, an average clock signal error (CS_(ERR)) can be determined based on the individual clock signal errors (CS_(ERR)) determined for each test piece. Using the exemplary calibration values presented above, the average CS_(ERR) is equal to +0.00333 based on the following equation:

$\begin{matrix} {{C\; S_{ERR}} = \frac{\sum\limits_{i = 1}^{n}{C\; S_{ERR}}}{n}} & (4) \end{matrix}$

At step 650, a clock signal error factor (CSF_(ERR)) can be determined based on the clock signal error (CS_(ERR)) divided by the difference between the second calibration temperature (T_(C2)=+85° C.) and the first calibration temperature (T_(C1)=−40° C.) (i.e., +125° C.). Using the exemplary calibration values presented above, the clock signal error factor (CSF_(ERR)) is equal to +0.00002664/° C. based on the following equation:

$\begin{matrix} {{C\; S\; F_{ERR}} = {\frac{C\; S_{ERR}}{\Delta\; T} = \frac{C\; S_{ERR}}{T_{C\; 2} - T_{C\; 1}}}} & (5) \end{matrix}$

At step 660, a measured time of flight (TOF_(M)) is measured using the ultrasonic testing unit 120. At step 670, the temperature (e.g., T_(M)=+60° C.) of the ultrasonic testing unit 120 (e.g., the clock oscillator 132) is measured by a temperature measurement device 140 of the ultrasonic testing unit 120. At step 680, the ultrasonic testing unit 120 can determine a clock signal corrected time of flight (TOF_(CSC)) based on the clock signal error factor (CSF_(ERR)) and the difference between the normalized calibration temperature (T_(NC)) and the measured temperature (T_(M)) in the following equation: TOF_(CSC)=TOF_(M)*(1+(T _(NC) −T _(M))*CSF _(ERR)))  (6) Where TOF_(CSC)=clock signal corrected time of flight, TOF_(M)=measured time of flight, T_(NC)=normalized calibration temperature (e.g., +25° C.), T_(M)=measured temperature of the ultrasonic testing unit 120 (e.g., +60° C.), and CSF_(ERR)=clock signal error factor (e.g., +0.00002664/° C.).

Using the exemplary values presented above, the clock signal corrected time of flight (TOF_(CSC)) for the measured temperature (T_(M)) of +60° C. would be 0.09324% less than the measured time of flight (TOF_(M)) to adjust for the greater number of clock pulses resulting from the higher temperature. Similarly, when the ultrasonic testing unit 120 is operating at a measured temperature (T_(M)) of −15° C., the clock signal corrected time of flight (TOF_(CSC)) would be 0.1066% greater than the measured time of flight (TOF_(M)) to adjust for the lesser number clock pulses resulting from the lower temperature. In another embodiment, rather than being applied to a measured time of flight (TOF_(M)), the clock signal error factor (CSF_(ERR)) can be applied to the pulser corrected time of flight (TOF_(PC)), which already includes the correction for the pulse delay variation caused by the variation in temperatures of the ultrasonic testing unit.

In view of the foregoing, embodiments of the invention provide more accurate time of flight measurements that correct for temperature variation of the ultrasonic testing unit. A technical effect is to provide more accurate thickness measurements and to more accurately identify corrosion.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method, or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.), or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “service,” “circuit,” “circuitry,” “module,” and/or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon.

Any combination of one or more computer readable medium(s) may be utilized. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable storage medium may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing. In the context of this document, a computer readable storage medium may be any tangible medium that can contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device.

Program code and/or executable instructions embodied on a computer readable medium may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc., or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The program code may execute entirely on the user's computer (device), partly on the user's computer, as a stand-alone software package, partly on the user's computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user's computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider).

Aspects of the present invention are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer readable medium that can direct a computer, other programmable data processing apparatus, or other devices to function in a particular manner, such that the instructions stored in the computer readable medium produce an article of manufacture including instructions which implement the function/act specified in the flowchart and/or block diagram block or blocks.

The computer program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other devices to cause a series of operational steps to be performed on the computer, other programmable apparatus or other devices to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

What is claimed is:
 1. A method for correcting a measured time of flight measured by an ultrasonic testing unit for varying pulse delays caused by varying temperatures of the ultrasonic testing unit, the method comprising the steps of: calibrating the ultrasonic testing unit at a normalized calibration temperature by determining a normalized time of flight for a known material and a known thickness; determining a first pulse delay at a first calibration temperature of the ultrasonic testing unit; determining a second pulse delay at a second calibration temperature of the ultrasonic testing unit; and determining a time of flight pulse delay offset based on the difference between the second pulse delay and the first pulse delay and the difference between the second calibration temperature and the first calibration temperature.
 2. The method of claim 1, further comprising the steps of: measuring the temperature of the ultrasonic testing unit; and determining a pulser corrected time of flight based on the time of flight pulse delay offset and the difference between the normalized calibration temperature of the ultrasonic testing unit and the measured temperature of the ultrasonic testing unit.
 3. The method of claim 2, wherein the step of measuring the temperature of the ultrasonic testing unit comprises measuring the temperature of a logic circuit of the ultrasonic testing unit.
 4. The method of claim 2, wherein the pulser corrected time of flight is determined by subtracting from the measured time of flight the product of the time of flight pulse delay offset and the difference between the normalized calibration temperature of the ultrasonic testing unit and the measured temperature of the ultrasonic testing unit.
 5. The method of claim 1, wherein the first calibration temperature is lower than the normalized calibration temperature.
 6. The method of claim 1, wherein the second calibration temperature is higher than the normalized calibration temperature.
 7. The method of claim 1, wherein the step of determining a time of flight pulse delay offset comprises dividing the difference between the second pulse delay and the first pulse delay by the difference between the second calibration temperature and the first calibration temperature.
 8. An ultrasonic testing system for inspecting a test object, the ultrasonic testing system comprising: an ultrasonic probe; and an ultrasonic testing unit connected to the probe, the ultrasonic testing unit comprising a logic circuit, a pulser, a clock oscillator, a temperature measurement device, and a microcontroller comprising executable instructions for calibrating the ultrasonic testing unit at a normalized calibration temperature by determining a normalized time of flight for a known material and a known thickness; determining a first pulse delay at a first calibration temperature of the ultrasonic testing unit; determining a second pulse delay at a second calibration temperature of the ultrasonic testing unit; and determining a time of flight pulse delay offset based on the difference between the second pulse delay and the first pulse delay and the difference between the second calibration temperature and the first calibration temperature.
 9. The ultrasonic testing system of claim 8, wherein the logic circuit is one of a field programmable gate array (FPGA) or an application specific logic circuit (ASIC).
 10. The ultrasonic testing system of claim 8, wherein the temperature measurement device is a thermistor.
 11. The ultrasonic testing system of claim 8, wherein the temperature measurement device is a thermocouple.
 12. A method for correcting a measured time of flight measured by an ultrasonic testing unit for varying clock signal frequencies caused by varying temperatures of the ultrasonic testing unit, the method comprising the steps of: calibrating the ultrasonic testing unit at a normalized calibration temperature by determining a normalized time of flight for a known material and a known thickness; determining a first time of flight for a first test piece of a known material and a first known thickness at a first calibration temperature of the ultrasonic testing unit; determining a second time of flight for the first test piece at a second calibration temperature of the ultrasonic testing unit; determining a clock signal error based on the difference between the second time of flight for the first test piece and the first time of flight for the first test piece; and determining a clock signal error factor based on the clock signal error and the difference between the second calibration temperature and the first calibration temperature.
 13. The method of claim 12, further comprising the steps of: measuring the temperature of the ultrasonic testing unit; and determining a clock signal corrected time of flight based on the clock signal error factor and the difference between the normalized calibration temperature of the ultrasonic testing unit and the measured temperature of the ultrasonic testing unit.
 14. The method of claim 13, wherein the step of measuring the temperature of the ultrasonic testing unit comprises measuring the temperature of a clock oscillator of the ultrasonic testing unit.
 15. The method of claim 13, wherein the clock signal corrected time of flight is determined by multiplying the measured time of flight by the sum of one plus the product of the clock signal error factor and the difference between the normalized calibration temperature of the ultrasonic testing unit and the measured temperature of the ultrasonic testing unit.
 16. The method of claim 12, wherein the first calibration temperature is lower than the normalized calibration temperature.
 17. The method of claim 12, wherein the second calibration temperature is higher than the normalized calibration temperature.
 18. The method of claim 12, wherein the step of determining a clock signal error comprises dividing the difference between the second time of flight for the first test piece and the first time of flight for the first test piece by the first or second time of flight.
 19. The method of claim 12, wherein the step of determining a clock signal error factor comprises dividing the clock signal error by the difference between the second calibration temperature and the first calibration temperature.
 20. The method of claim 12, further comprising the steps of: determining a first time of flight for a second test piece of the known material and a second known thickness at the first calibration temperature of the ultrasonic testing unit; determining a second time of flight for the second test piece at the second calibration temperature of the ultrasonic testing unit; determining a first time of flight for a third test piece of the known material and a third known thickness at the first calibration temperature of the ultrasonic testing unit; and determining a second time of flight for the third test piece at the second calibration temperature of the ultrasonic testing unit, wherein the clock signal error is determined based on the average of the difference between the second time of flight for the first test piece and the first time of flight for the first test piece, the difference between the second time of flight for the second test piece and the first time of flight for the second test piece, and the difference between the second time of flight for the third test piece and the first time of flight for the third test piece. 